The Nusra Front, al-Qaeda's affiliate in Syria, captured most of the northwestern city of Idlib from government forces Saturday, sweeping into neighborhoods in the center of the city in a powerful blow to President Bashar Assad's government, opposition activists and the group said.

The Nusra Front is leading a group of ultra-conservative rebels in a major offensive that began earlier this week to take Idlib, near the border with Turkey. If the city falls, it would be the second provincial capital and major urban center lost by President Bashar Assad's forces in the four-year-old conflict.

Nusra Front forces in Idlib (Photo: Reuters)

The eastern city of Raqqa fell to rebels, including the Nusra Front, in March 2013. It was subsequently taken over by Nusra's rival, the Islamic State group, which has since declared the city its de facto capital.

The Britain-based Syrian Observatory for Human Rights said rebel fighters seized control of most of Idlib in a push Friday evening and early Saturday after collapsing government forces withdrew to their bases and several other buildings in the city.

The group, which relies on an extensive network of activists across Syria, said heavy fighting continued Saturday amid heavy artillery shelling from both sides. The Local Coordination Committees, another opposition activist collective in Syria, also reported the "almost complete" capture of large parts of Idlib by the rebels.

The Islamist alliance calls its operation Army of Fatah, a reference to the conquests that spread Islam across the Middle East from the seventh century.

Footage posted on the internet earlier on Saturday by Nusra Front showed dozens of fighters in the streets of what a voiceover said was Idlib city.

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"This is my house, for four years I have not entered it. This is my neighbourhood, this is our country and by God's will we will liberate it and settle Muslims in it," a fighter said in the video.

He was welcomed by several men. Some embraced him and wept.

In another video, fighters were shown on top of a building which bore the insignia of Assad's Baath party, trying to tear down huge posters of the president.

An unnamed Syrian military official quoted by state-run news agency SANA said army forces were fighting "fierce battles" against "armed terrorist groups" to regain control in Idlib, adding that the opposing side suffered heavy losses. In an indication of the gravity of the situation, SANA also said the army is repositioning forces and units in the city to face "thousands of terrorists crossing the border from Turkey."

Earlier, the agency said army troops directed strikes at fighters who tried to sneak into the area near the national museum and the industrial zone around the eastern entrance of Idlib.

Civilians returning to Idlib (Photo: Reuters)

The government claimed earlier this week that "thousands of terrorists" streamed in from Turkey to attack Idlib and its suburbs. Turkey is one of the main backers of the rebels.

A loose alliance of anti-government rebels and members of the Nusra Front control large parts of Idlib province and have tried in the past to capture the provincial capital. The latest attempt appears to be the most successful so far.

On its Twitter account, Nusra said its fighters have taken control of half of the city, posting pictures of the Clock Tower and other landmark locations now under the its control.

In one video, also posted by Nusra on Twitter, a rebel fighter is seen tearing a billboard in Idlib's municipal stadium showing Assad in a suit and tie, with the words "Together, we protect it (Syria)," before trampling it with his feet. The video, which could not be independently confirmed, appeared consistent with AP reporting on events in Idlib.

The humiliating losses in Idlib mark the second blow to government forces this week, after rebels, led by Nusra, captured the ancient and strategic town of Busra Sham in southern Syria.

Also Saturday, UN Secretary-General Ban Ki-moon said he was angry and shamed by the failure of the world to stop Syria's raging civil war. He promised to step up diplomatic efforts in comments at a summit of Arab leaders in the Red Sea resort city of Sharm el-Sheikh in Egypt.

More than 220,000 people have been killed in the conflict, which began with popular protests amid Arab Spring uprisings in March 2011 and turned into an insurgency following a brutal military crackdown.

Syria's seat was empty at the Arab League summit in Sharm this weekend. The main Western-backed Syrian group, the Syrian National Council, slammed the Arab League's decision not to invite it for the second year in a row.

Rebels in Syria (Photo: Reuters)

The group considers itself as the legitimate representative of the Syrian people and had been declared as such by the Arab League. But its role has diminished greatly in the past year as militant groups became increasingly powerful in Syria.

In a statement, the Syrian National Coalition said the non-invite coincided with attempts to "float the (Syrian) regime" – a clear reference to US Secretary of State John Kerry's recent statement that Washington must eventually talk with Damascus to help negotiate an end to Syria's civil war.

Assad addressed those comments in an interview with CBS aired Friday in which he said Syria is open to dialogue "with anyone, including the United States." He said there is no direct communication so far with Washington.